Relationship Between Tornado Debris Signature (TDS) Height

and Tornado Intensity

Chad EntremontDaniel Lamb

NWS Jackson, MS

Overview• Purpose of the study• What defines a tornado debris

signature (TDS)• Background on the data for the study• Results & Findings• Example case:

Apr 11, 2013• Other TDS research

Purpose• Initial study - to determine if there is a

relationship between TDS height and tornado intensity

• Results/findings show a strong correlation– Can enhance tornado warning wording

when lacking spotter/visual confirmation• Use “confirmed tornado” wording• Impact Based Warnings (IBW)• Considerable & Catastrophic Tags

– Develop a regression equation to provide a “first guess” on tornado intensity

Motivation• Impetus for this study started with dual-

pol training from WDTB• NWA, Jan 2012 - presentations on Apr 27,

2011 outbreak and TDS detection• Furthered by local training and

investigation of the TDS• Recent papers by:

– Dual-Polarization Tornadic Debris Signatures Part I: Examples and Utility in an Operational Setting (Schultz et al. 2012.)

– Tornado Damage Estimation Using Polarimetric Radar (Bodine et al. 2013)

Defining a TDS1. Identify a valid

velocity circulation2. Low correlation

coefficient (CC) <.90 collocated with circulation

3. Sufficient reflectivity >35 dbZ collocated with # 1 & 2

4. Lowering of differential reflectivity (ZDR) near 0

Determining TDS Height (Maximum)

0.5°

0.64 @ 1200 ft

Determining TDS Height (Maximum)

0.9°

0.64 @ 1200 ft0.75 @ 2020 ft

Determining TDS Height (Maximum)

1.4°

0.64 @ 1200 ft0.75 @ 2020 ft0.79 @ 3090 ft

Determining TDS Height (Maximum)

1.9°

0.64 @ 1200 ft0.75 @ 2020 ft0.79 @ 3090 ft0.94 @ 4060 ft

Determining TDS HeightHattiesburg, MS

0.5°

0.26 @ 6300 ft

Determining TDS HeightHattiesburg, MS

0.9°

0.45 @ 9000 ft

Determining TDS HeightHattiesburg, MS

1.3°

0.54 @ 12,100 ft

Determining TDS HeightHattiesburg, MS

1.8°

0.77 @ 15,700 ft

Determining TDS HeightHattiesburg, MS

2.4°

0.81 @ 20,000 ft

Determining TDS HeightHattiesburg, MS

3.1°

0.77 @ 24,500 ft

Determining TDS HeightHattiesburg, MS

4.0°

0.76 @ 30,600 ft

Dataset Information• Data consisted of– TDS cases from 2010 to present– Central Plains through Florida– GR2 Analyst used to analyze the hi-res

data

• Sample size n = 181– 54 QLCS cases– 127 Supercell cases (3 Tropical )

• 5 to 10 cases not used do to poor data and uncertainty in the true TDS height

Dataset Information

EF-0 EF-1 EF-2 EF-3 EF-4 EF-524 62 51 27 14 3

Data stratified by EF scale

Scatter PlotAll TDS Cases

Scatter PlotSupercell vs QLCS

PercentilesBox (25th – 75th )

PercentilesStratified by Storm Mode

Regression

R = .76

Results

• Data supports a strong correlation for maximum TDS height vs tornado intensity– Especially for strong/violent tornadoes– 10 kft is a critical level– Stronger correlation exists with

supercells than with QLCS type tornadoes

• More likely to see a TDS from strong/violent tornadoes at greater distance from the radar with a distinct TDS height correlation

Results

• No skill differentiating EF-0 & EF-1• Weaker tornadoes are typically seen

within 50 nm of the radar with the max TDS height below 10 kft

• Regression indicates a solid correlation and provides a “first guess” of tornado intensity based off max TDS height

Example CaseApril 11, 2013

• 68 mi EF3• Poorly anticipated

event• No watch (for the

counties effected)• Due to lack of

spotters/chasers, dual pol data was critical for decision making

Example CaseApril 11, 2013

• Tor Warning in effect• TDS identified @

1640z (Tornado started @ 1635z)

• Mentioned “confirmed tornado” in warning product

Example CaseApril 11, 2013

• Radar vol scan sequence• Max TDS heights per vol scan:– 1640z 11.9 kft

• SVS mentioned “confirmed tornado”

– 1645z 12.6 kft• SVS “confirmed tornado” “possibly

strong”

– 1650z 17.2 kft• Warning team now considering “Tor

Emergency”

Example CaseApril 11, 2013

• Max TDS height per vol scan:– 1654z 14 kft

• Vel increase – gtg 115kts

– 1659z 10.7 kft• Vel increase – gtg 137kts• Decision made to upgrade to “Tor

Emergency”

– 1704z 15.1 kft• Best defined TDS (per CC data)• Start of the most consistent intense

damage (EF3)

Example CaseApril 11, 2013

• 1704z vol scan

Example CaseApril 11, 2013

Summary:• Initial TDS was >10

kft• Tornado quickly

reached “strong” category

• TDS >10 kft was maintained for entire path

Additional Findings• Determining a TDS & max height can be a

challenge when close (<15 nm) from radar– Due to well defined “hook echo” and lack of

sufficient reflectivity associated with WER– Process of viewing many (if not all) elevation slices,

especially with fast moving lines/cells– Dealing with the often “noisy” data, numerous/small

radar range bins– TDS can reach above 19.5 elev

• Due to the split cut mode (88D VCP 12 lowest 3 tilts) the TDS has a slight offset from the Vel data (typically 15sec) as the Z, CC scans occur before the Vel scans) * strongly related to storm speed

Additional Findings• TDSs typically show up 1-3 vol scans after

tornado touchdown– Critical height levels (>10kft) are often met 1 vol

scan after initial TDS for the stronger tornadoes

• Slower more erratic moving tornadoes typically make up the cases where TDS height falls below the 25th percentile

• ZDR often shows the full spectrum of values (+6 to -3 db) within the TDS

• TDSs may be masked by non-uniform beam filling, makes for an added detection challenge

Additional Research

• Incorporate landscape type (forest, urban area etc…)– Have started, this is complex–May work best where land type is

consistent or during the initial stages of the TDS before land types get mixed

• Continue to add cases to the database–Will strengthen the percentile results–Make the regression more robust